In existing data processing systems, magnetic tape is frequently utilized for storage of binary information because the tape has great storage capacity. The tape is wound on reels which are replaced when a read or write operation associated with data contained thereon is completed. The tape travels in an intermittent manner, in opposite directions, before an array of data read/write heads. The tape travels during a series of constant speed winding acceleration and deceleration intervals, with read/write operations being performed regardless of the tape travel direction.
Structures for winding such magnetic tape are well-known. One exemplary structure is disclosed in French Pat. No. 1,393,683, filed Mar. 25, 1964, by AMPEX Corporation with the title "Magnetic Tape Winding Device." The apparatus described in this patent includes first and second reels for carrying the tape, which reels may either be feed or takeup reels, in combination with two vacuum chambers for temporarily storing the tape, a single driving capstan positioned between the two chambers, and means for guiding the tape along a path between the reels, chambers and capstan. Each reel is equipped with a separate drive motor directly coupled to it.
The vacuum chambers have substantially constant cross-section and are connected to a vacuum source. The length of the vacuum chamber is much greater than the width thereof. In each vacuum chamber, the tape forms a loop that is positioned between one of the two reels and the capstan. The two reels are symmetrical with respect to the capstan, as well as the vacuum chambers.
Thus, each chamber includes a part situated between the tape loop and a closed wall thereof. This part of the chamber is referred to as the lower part of the chamber. Each chamber also includes a part that is situated between the loop and an open end of the chamber. The tape passes through the open end of the chamber, which is referred to as the upper part of the chamber. A vacuum source is connected in fluid flow relation with the lower part of the chamber. The pressure P.sub.i in the lower part of the chamber is lower than the pressure P.sub.s in the upper part of the chamber; generally the upper part of the chamber is at atmospheric pressure. As a result of this construction, it is possible to start and stop the tape in a period on the order of a few milliseconds.
To measure the displacement of the tape relative to the reels and heads, it is necessary to detect the position of the loop within each vacuum chamber at any instant. By determining the position of the tape loop in the vacuum chamber at any instant, the length of the portion of the tape present in the vacuum chamber is ascertained. To enable the position of the tape loop to be determined, each vacuum chamber includes a means for detecting the position of the tape loop contained therein. The tape loop position detector derives an electric, generally analog, signal having a magnitude that is a function of the tape loop position and therefore of the length of tape contained in the vacuum chamber. The electric signal is supplied to an electronic device for controlling a drive motor of a reel for the tape supplied to the vacuum chamber that includes the detector.
A detector of this general type is disclosed, for example, in French Pat. No. 1,362,356, filed July 9, 1963, by Nippon Electric Company, Ltd., with the title "Apparatus for Manipulating Tapes, For Example Recording Tapes." The structure disclosed in this patent includes a slot formed in an electrically grounded, metal wall of the chamber. The slot is positioned so that it is parallel to the length of the wall and to the tape surface in the chamber. To capacitively monitor the tape loop position, a moveable and deformable flexible strip made of a plastic, dielectric material, such as Mylar, coated by a thick metal sheet or foil is situated outside of the vacuum chamber. A part of the flexible, plastic strip faces the slot and may contact the slot and the wall containing the slot. In contrast, the metal coating on the strip does not contact the slot. The moveable and deformable flexible strip is biased by a vacuum source, having a pressure P.sub.c that is between P.sub.i and P.sub.s.
Because of the slot, the relatively high vacuum established in the lower part of the vacuum chamber biases a first end of the flexible strip directly against a wall in the lower part of the vacuum chamber. In the upper part of the chamber, above the loop, where atmospheric pressure generally subsists, the vacuum source biases a second end of the flexible strip against a metal stop plate that is remote from and lies in a plane parallel to the chamber wall. This is because the pressure P.sub.c of the vacuum source is less than the generally atmospheric pressure P.sub.s in the upper part of the chamber. The chamber wall is a grounded metal wall, that forms a fixed electrode of a capacitor. The metal strip or foil on the flexible strip forms a moveable electrode of the capacitor. The plastic strip is located between the two electrodes and thus forms a dielectric of the capacitor. The capacitance between the electrodes is a function of an inflection or deflection region of the strip between the wall and the metal stop plate. The position of the strip inflection or deflection is in turn a function of the tape loop position, and therefore of the length of the magnetic tape portion contained in the vacuum chamber.
In current practice, the slot may be replaced by a series of holes situated along side each other in the same direction as the slot, i.e, parallel to the length of the vacuum chamber. Thus, the slot or series of holes forms a means for communicating between the vacuum chamber and an exterior environment.
Variations in the capacity of the capacitive detector, as a function of tape loop position in the vacuum chamber, are converted into analog electric signal variations. The analog signal is amplified by an electronic amplifying device associated with the detector to drive the motor, as stated supra.
A detector of the type described, while being easy to manufacture, has certain disadvantages. One of the disadvantages is that the thickness of the Mylar dielectric is usually quite small, whereby air penetrating through the slot in the upper section of the chamber causes the dielectric and the wall adjacent the dielectric to be soiled by several impurities, such as dust. Such soiling causes the thickness of the dielectric to vary as a function of time, to impair the detector accuracy. The moveable, flexible, thin electrode formed by a coating on the plastic strip also has a tendency to be deformed because of the suction exerted thereon by the relatively high negative pressure extant in the lower section of the vacuum chamber. Deformation of the flexible, thin electrode causes the edges of the electrode to be improperly in register with a wall of the vacuum chamber adjacent the slot. Thereby, leakage may result between the edges of the flexible electrode and the vacuum chamber wall adjacent the slot, to prevent deflection of the electrode at the tape loop position with a resulting inaccuracy of the detected capacitance.